This invention relates to a tripping device which is incorporated in an overload circuit breaker and which has a helically bent bimetal strip attached to the housing of the circuit breaker. One metal component of the bimetal strip is arranged at the inside of the helix, while the other metal component is arranged at the outside thereof. In prior art structures one end of the bimetal helix is affixed to the housing, while its other, free terminus conventionally constitutes the portion (tripping portion) which executes the tripping motion.
In tripping devices of the above-outlined type the current terminal connected with the free end of the bimetal helix is constituted by a flexible (braided) wire to ensure that it does not obstruct the free mobility of the bimetal helix. A disadvantage of known helically bent bimetal strips resides in the fact that the tripping motion of the free helix end is, because of the instability of the bimetal helix, particularly in the zone of its free end, can be controlled (guided) in a localized manner only with difficulty along a predetermined path of motion. These difficulties increase as the axial length of the bimetal helix is augmented, that is, as the number of turns of the helix increases. When an overload circuit breaker is used in relatively low-current intensity circuits, the heat generated by the excess current as it passes through a planar bimetal strip may not be sufficient for tripping. Thus, for increasing the electrical resistance (and thus the generated heat) of the bimetal strip, for example, its effective length has to be increased within a very limited space. In such instances helically wound bimetal strips find advantageous application. An appreciable extension of the effective length of the bimetal strip in a narrow space, that is, in case of a small helix diameter, however, requires a certain minimum number of turns and thus the instability of the bimetal strip (which is a direct function of the number of turns) increases precisely at the tripping terminus of the helix.